In a vehicle where an engine (internal combustion engine) is mounted, an automatic transmission automatically and optimally setting a gear ratio between the engine and a drive wheel is known as a transmission for suitably transmitting torque and rotation speed generated by the engine to the drive wheel in accordance with a driving condition of the vehicle.
For example, a planetary gear type transmission setting a gear ratio (gear position) using a clutch, a brake, and a planetary gear device, and a belt-type continuously variable transmission (CVT) performing stepless adjustment of a gear ratio exist as an automatic transmission mounted in a vehicle.
A belt-type continuously variable transmission has a belt wound around a primary pulley (input-side pulley) and a secondary pulley (output-side pulley) having a pulley groove (V-groove) and is configured such that a gear ratio is set in a stepless fashion by simultaneously widening a groove width of the pulley groove of one of the pulleys and narrowing a groove width of the pulley grove of the other pulley so as to continuously vary a winding radius (effective diameter) of the belt with respect to each of the pulleys. A torque transmitted in this belt-type continuously variable transmission is a torque corresponding to a load acting in a direction in which the belt and the pulleys are made to contact mutually, and therefore, in order to apply tension to the belt, the belt is clamped by the pulleys.
Furthermore, as explained above, a shift transmission is carried out in a belt-type continuously variable transmission by widening and narrowing the groove widths of the pulley grooves. Specifically, each of the primary pulley and the secondary pulley comprises a fixed sheave and a moveable sheave, and a shift transmission is carried out by moving the moveable sheave forwards and backwards in an axial direction using a hydraulic actuator provided at a rear face side thereof.
In this way, in a belt-type continuously variable transmission, the belt is clamped by the pulleys in order to apply tension to the belt, and in addition, the condition of clamping of the belt by the pulleys is changed in order to carry out a shift transmission. Accordingly, the groove width of the primary pulley and the groove width of the secondary pulley are simultaneously changed by delivering hydraulic pressure corresponding to a required torque as typified by engine load, etc. to the hydraulic actuator at the secondary pulley side so as to secure a necessary transmission torque capacity and by delivering hydraulic pressure for carrying out a shift transmission to the hydraulic actuator at the primary pulley side.
Furthermore, in a vehicle where an automatic transmission is mounted, a fluid-type transmission device such as a fluid coupling or torque converter, etc. is disposed between the engine and the automatic transmission. A fluid-type transmission device provided with a lock-up clutch directly connecting an input side and an output side of the fluid-type transmission device through frictional engagement using an oil pressure of an operating oil exists as a fluid-type transmission device.
Furthermore, in a vehicle where this type of fluid-type transmission device featuring a lock-up clutch is mounted, engagement and disengagement of the lock-up clutch is performed by controlling a hydraulic pressure made to act on the lock-up clutch with, for example, a hydraulic pressure (line pressure) of a hydraulic control system including hydraulic control of an automatic transmission used as an initial pressure (for example, see patent documents 1 and 2). Specifically, in a case of a torque converter featuring a lock-up clutch, control of engagement and disengagement of the lock-up clutch is performed by controlling a differential pressure (lock-up differential pressure) between an engagement-side pressure chamber and a disengagement-side pressure chamber of the torque converter using a lock-up differential-pressure control solenoid valve and a lock-up control valve, etc. and based on a lock-up differential pressure instruction value.
In certain cases in the control of a lock-up clutch, deceleration lock-up control controlling engagement of the lock-up clutch is performed upon deceleration with the accelerator off. With deceleration lock-up control of this type, in order to, for example, prevent stalling of the engine as a result of a reduction in vehicle speed, rapid disengagement of the lock-up clutch is made possible by maintaining engagement at a lowest possible differential pressure (a low-pressure engagement pressure within a range where slipping does not occur) capable of withstanding negative torque such as an auxiliary machinery load and friction of the engine when the accelerator is off (when non-driven), etc.
Furthermore, in control of the lock-up clutch, lock-up smooth off control is executed upon the completion of deceleration lock-up control. Lock-up smooth off control is control for disengaging the lock-up clutch as quickly as possible while suppressing disengagement shock upon deceleration lock-up control completion. In specific terms, it is a control that, at a time whereat there was a completion instruction (lock-up clutch disengagement instruction) of deceleration lock-up control, sets a disengagement initial pressure of lock-up smooth off control based on the vehicle speed, etc., gradually decreases the lock-up differential pressure from that disengagement initial pressure at a prescribed sweep gradient (constant rate of change), and smoothly disengages the lock-up clutch.
It should be noted that, although learning correction of a deceleration lock-up differential pressure (low-pressure engagement pressure) is desirable during deceleration lock-up control, the lock-up clutch needs to be reliably maintained in an engagement condition during deceleration lock-up control, and therefore, execution of feedback control of the deceleration lock-up differential pressure and performing learning correction is difficult.
For this reason, in conventional control, in order that the lock-up clutch does not adopt a slip condition despite disparity in a hydraulic pressure characteristic of the lock-up differential-pressure control solenoid valve controlling the lock-up differential pressure or disparity in hydraulic pressure control due to other individual differences, etc., the deceleration lock-up differential pressure during deceleration lock-up control is set a little larger in consideration of the hydraulic pressure disparity. This point is hereinafter explained with reference to FIG. 9.
First of all, as shown in FIG. 9, in a case where, in contrast to a hydraulic pressure characteristic in a case where hydraulic-pressure control components such as the lock-up differential-pressure control solenoid valve are nominal items (i.e., a hydraulic pressure characteristic as shown by a solid line in the figure), a prescribed disparity (tolerance) as shown by a dashed line in the figure exists, it is necessary to assume lower limit items corresponding to the lowest engagement hydraulic pressure and to set the deceleration lock-up differential pressure a little larger in order to avoid slipping of the lock-up clutch. Specifically, if a map with nominal items as standard as shown by the solid line in FIG. 9 is set as a conversion map for calculating a lock-up differential pressure instruction value PD based on a target value of a lock-up differential pressure PLU, in a case where the hydraulic pressure characteristic corresponds to lower limit items, when the lock-up differential pressure PLU (the target value with nominal items as standard) is [c], the lock-up differential pressure instruction value PD becomes [b]; however, the actual lock-up differential pressure PLU may become a value [a] lower than [c] and the lock-up differential pressure PLU may be insufficient. In conventional control, in order to avoid this, the lock-up differential pressure PLU (the target value with nominal items as standard) is set to a value larger than [a] by a hydraulic pressure disparity correction amount PE.
In a case where the lock-up differential pressure PLU is set a little larger in this way, if the actually-mounted hydraulic-pressure control components such as the lock-up differential-pressure control solenoid valve are nominal items, the actual lock-up differential pressure PLU remains to be hydraulic pressure [c], and if the components are upper limit items, the lock-up differential pressure PLU becomes an even higher hydraulic pressure [d], resulting in control to a higher hydraulic pressure than required. Furthermore, in a case where a conversion map has been set with the hydraulic pressure characteristic of lower limit items as standard, there is no need for the target value of the lock-up differential pressure PLU itself to be made large, but the actual hydraulic pressure becomes larger than required with nominal items and upper limit items.
With the object of eliminating such issues, the applicant of the present invention proposes control appropriately setting the deceleration lock-up hydraulic pressure (low-pressure engagement pressure) during deceleration lock-up control regardless of disparity, etc. of the hydraulic-pressure characteristic of the lock-up clutch.
With this proposed technology, when lock-up smooth off control, gradually disengaging the lock-up clutch upon the completion of deceleration lock-up control, is executed, a disengagement initial pressure of lock-up smooth off control is learned, and the deceleration lock-up differential pressure of deceleration lock-up control is updated to reflect this learning value of the disengagement initial pressure. By executing such deceleration lock-up differential pressure learning control, the deceleration lock-up differential pressure of deceleration lock-up control can be appropriately set in accordance with the hydraulic pressure characteristic, etc. of the actually-mounted lock-up differential-pressure control solenoid valve. That is to say, as the hydraulic pressure characteristic, etc. of the actually-mounted lock-up differential-pressure control solenoid valve is reflected in a disengagement initial pressure learning value of lock-up smooth off control, by updating the deceleration lock-up differential pressure of deceleration lock-up control to reflect that disengagement initial pressure learning value, the deceleration lock-up differential pressure can be appropriately lowered while avoiding a slip condition of the lock-up clutch during deceleration lock-up control.
Furthermore, a learning technology whereby learning is performed until a control differential pressure is reached by gradually changing an initial lock-up differential pressure in a direction of disengagement in accordance with a number of repetitions of deceleration lock-up control has been proposed as a technology for lowering the control hydraulic pressure of deceleration lock-up control (for example, see Patent Citation 1).
Patent Citation 1: JP 2004-124969A
Patent Citation 2: JP H05-180327A
Patent Citation 3: JP H10-159967A
Patent Citation 4: JP H09-196158A
Patent Citation 5: JP H07-027219A